1. Field of the Invention
The present invention relates to a drive circuit for a motor rotated in synchronization with synchronizing pulse signals, or a so-called pulse synchronized motor.
2. Description of the Prior Art
Such drive circuit for a pulse synchronized motor is utilized, for example, in pulse synchronized projectors or in pulse synchronized movie cameras. In a pulse synchronized projector, the pulse signals determining the projecting speed of film are recorded simultaneously with acoustic signals on an acoustic recording device such as a tape recorder, and thus recorded pulse signals are used at projection, simultaneously with the playback of said acoustic signals, for controlling the rotation of the motor of said projector thereby synchronizing the projected image with the acoustic signal. Also pulse synchronized movie cameras are used for a case of synchronized picture taking with plural cameras, for example three-dimensional motion-picture taking with two cameras or motion-picture taking from plural positions of a phenomenon for the purpose of observing time-dependent change thereof wherein the frames of the films in plural cameras have to be exposed at the same time. For such drive motors there has been proposed a drive circuit as shown in FIG. 1.
Referring to FIG. 1, 1 indicates a flip-flop circuit or a similar bistable circuit provided with two input terminals S and R, and a control output terminal O. A motorcontrol transistor 2 controls the supply of electric current to a direct current motor 3 according to the output signal Sig-3 of said control output terminal O of the bistable circuit 1. Said motor 3 is supplied with an electric current from a direct current supply E.sub.B. Switch 4 is mechanically connected with the rotation of said motor 3 and performs on-off operations of a number proportional to the rotation of said motor 3. A differentiating circuit 5 generates motor speed pulse signals Sig-2 in response to the on-off operations of said switch 4. Thus, upon application of a synchronizing pulse signal Sig-1 to said input terminal S, said bistable circuit shifts to `set` state to release an `on` signal to said transistor 2 thereby permitting the supply of electric current to said motor 3, and upon receipt of a motor speed pulse signal Sig-2 said bistable circuit 1 shifts to `reset` state to release an `off` signal to the transistor 2 thereby interrupting the supply of electric current to said motor 3. By the suitable setting of the voltage of the direct current supply source E.sub.B the cycles of said synchronizing pulse signals Sig-1 and that of said motor speed pulse signals Sig-2 are maintained in a particular mutual phase relationship as shown in FIGS. 2A, 2B and 2C, and the motor 3 attains an average rotation speed proportional to the synchronizing pulse signals Sig-1 to realize synchronized control.
In the synchronized control by means of a circuit as explained above, there stands a certain relationship between the frequency of synchronizing pulse signals Sig-1 and the voltage V.sub.m applied to the motor 3 as shown in FIG. 3, which represents these two variables in abscissa and ordinate, respectively. In case of continuously elevating said voltage V.sub.m with synchronizing pulse signals of a given frequency, the motor 3, which is in an asynchronous state with a low rotation speed while the voltage is excessively low, assumes the synchronized state beyond a certain voltage wherein the average rotation speed thereof is proportional to the frequency of synchronizing pulse signals, and this state is maintained even when the voltage V.sub.m is further elevated within a certain range. Upon further elevation of the voltage beyond a certain limit, the motor 3 again enters an asynchronous state with a rotation speed higher than that instructed by the synchronizing pulse signals. Thus, in order to synchronize the rotation of motor 3 with the synchronizing signals of a given frequency, it is necessary to control the voltage V.sub.m applied to said motor within a range determined by an upper limit and a lower limit.
The lines (a) and (b) in FIG. 3 respectively indicate the upper and lower limit voltages at various frequencies. Stated differently FIG. 3 indicates that the motor 3 is in an asynchronous state with a rotation speed higher or lower than that indicated by the synchronizing pulse signals respectively when the point corresponding to the given frequency of synchronizing pulse signals and to the applied voltage V.sub.m is located above the line (a) or below the line (b), and is in a synchronized state when said point is located within the range between the lines (a) and (b), namely the synchronizable voltage range. In case of the circuit of FIG. 1 wherein the voltage V.sub.m applied to the motor is equal to the supply voltage E.sub.B, a horizontal line (c) of voltage E.sub.B corresponding to the motor voltage crosses said lines (a) and (b) at the frequencies (d) and (e). Thus the portion between the lines (a) and (b), or defined by the upper and lower limit frequencies (d) and (e), represents the frequency range (f) synchronizable with the circuit of FIG. 1. In case of the circuit of FIG. 1, since a wide synchronizable frequency range is required for projectors or movie cameras, it becomes necessary to suitably regulate the motor voltage which is equal to the supply voltage E.sub.B if the synchronizing frequency is located outside said synchronizable frequency range (f). Also eventual fluctuation of supply voltage E.sub.B during the synchronized operation leads to a fluctuation of the motor voltage V.sub.m which may result in a shift from synchronized state to an asynchronous state.